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anastassius [24]
3 years ago
11

Which of the statements below does not describe antigens? a. The parts of antigen molecules that initiate immune responses are c

alled epitopes or antigenic determinants. b. Antigens can include proteins, nucleic acids, lipoproteins, glycoproteins, and certain large polysaccharides. c. Antigens exhibit immunogenicity and reactivity. d. Antigens only come from microbes.
Biology
2 answers:
Svet_ta [14]3 years ago
8 0

Answer:

D. antigen only come from microbes

Explanation:

Antigens are substances which elicits a response by the antibodies. Antigens Antigens include proteins, nucleic acids, lipoproteins, glycoproteins, and certain large polysaccharides.

They parts of antigen molecules that initiate immune responses are called epitopes or antigenic determinants.

Allushta [10]3 years ago
7 0

Answer:

The correct answer is d. Antigens only come from microbes. This statement does not describe antigens.

Explanation:

Our immune system produces antibodies when it detects harmful elements, called antigens. Antigens are substances (usually proteins), when introduced into an organism, trigger a defensive reaction. Thus, epitopes are the parts of a molecule that will be recognized by an antibody and to which it will bind. The vast majority of immunogens are proteins. These can be pure proteins or either glycoproteins or lipoproteins. Proteins are generally very good immunogens. Pure polysaccharides and lipopolysaccharides are good immunogens. Nucleic acids are generally weak immunogens. However, they can become immunogenic when they are formed by a single chain or when they are coupled to proteins. In general, lipids are not immunogenic but can behave like haptens. Antigens exhibit (or can show) a series of immunological properties including immunogenicity, which is the ability to stimulate an immune response, that is, to produce antibodies or a state of cellular immunity.

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Explain how we know that DNA breaks and rejoins during recombination.
alisha [4.7K]

Answer:

It occurs through homologous recombination

Explanation:

GENERAL RECOMBINATION OR HOMOLOGIST

           Previously we defined its general characteristics. We will now describe a molecular model of this recombination, based on the classic Meselson and Radding, modified with the latest advances. Do not forget that we are facing a model, that is, a hypothetical proposal to explain a set of experimental data. Not all points of this model are fully clarified or demonstrated:

           Suppose we have an exogenote and an endogenote, both consisting of double helices. In recombination models, the exogenote is usually referred to as donor DNA, and the endogenote as recipient DNA.

1) Start of recombination: Homologous recombination begins with an endonucleotide incision in one of the donor double helix chains. Responsible for this process is the nuclease RecBCD (= nuclease V), which acts as follows: it is randomly attached to the donor's DNA, and moves along the double helix until it finds a characteristic sequence called c

Once the sequence is recognized, the RecBCD nuclease cuts to 4-6 bases to the right (3 'side) of the upper chain (as we have written above). Then, this same protein, acting now as a helicase, unrolls the cut chain, causing a zone of single-stranded DNA (c.s. DNA) to move with its 3 ’free end

2) The gap left by the displaced portion of the donor cut chain is filled by reparative DNA synthesis.

3) The displaced single chain zone of the donor DNA is coated by subunits of the RecA protein (at the rate of one RecA monomer per 5-10 bases). Thus, that simple chain adopts an extended helical configuration.

4) Assimilation or synapse: This is the key moment of action of RecA. Somehow, the DNA-bound RecA c.s. The donor facilitates the encounter of the latter with the complementary double helix part of the recipient, so that in principle a triple helix is formed. Then, with the hydrolysis of ATP, RecA facilitates that the donor chain moves to the homologous chain of the receptor, and therefore matches the complementary one of that receptor. In this process, the chain portion of the donor's homologous receptor is displaced, causing the so-called "D-structure".

It is important to highlight that this process promoted by RecA depends on the donor and the recipient having great sequence homology (from 100 to 95%), and that these homology segments are more than 100 bases in length.

Note that this synapse involves the formation of a portion of heteroduplex in the double receptor helix: there is an area where each chain comes from a DNA c.d. different parental (donor and recipient).

5) It is assumed that the newly displaced chain of the recipient DNA (D-structure) is digested by nucleases.

6) Covalent union of the ends originating in the two homologous chains. This results in a simple cross-linking whereby the two double helices are "tied." The resulting global structure is called the Holliday structure or joint.

7) Migration of the branches: a complex formed by the RuvA and RuvB proteins is attached to the crossing point of the Holliday structure, which with ATP hydrolysis achieve the displacement of the Hollyday crossing point: in this way the portion of heteroduplex in both double helices.

8) Isomerization: to easily visualize it, imagine that we rotate the two segments of one of the DNA c.d. 180o with respect to the cross-linking point, to generate a flat structure that is isomeric from the previous one ("X structure").

9) Resolution of this structure: this step is catalyzed by the RuvC protein, which cuts and splices two of the chains cross-linked at the Hollyday junction. The result of the resolution may vary depending on whether the chains that were not previously involved in the cross-linking are cut and spliced, or that they are again involved in this second cutting and sealing operation:

a) If the cuts and splices affect the DNA chains that were not previously involved in the cross-linking, the result will be two reciprocal recombinant molecules, where each of the 4 chains are recombinant (there has been an exchange of markers between donor and recipient)

b) If the cuts and splices affect the same chains that had already participated in the first cross-linking, the result will consist of two double helices that present only two portions of heteroduplex DNA.

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3 years ago
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